Two-part adhesives provide high strength joints that require little, if any, machining to assemble. Typically, the two-part adhesive is made at or just before the time the adhesive must be applied to the mating surfaces of the joint. Appropriate quantities of the two parts are brought into contact and mixed thoroughly. Once mixed, the two-part fluid begins to harden (or cure or set) and must be applied to the joint before the cure becomes too advanced. Accordingly, the two-part adhesive is usually applied to one of the surfaces immediately and the two surfaces are clamped together for the required time to cure the adhesive. After the adhesive cures, the surfaces are unclamped and the assembled joint is used in higher level assemblies.
In some applications the adhesive joint must conduct either electricity, heat, or both electricity and heat while carrying a load. Because of the nature of the compounds formed in these two-part fluids, they typically do not have both the desired load carrying properties and the desired thermal or electric conductivity needed for various specific applications. To provide the desired conductivity, conductive particles are frequently introduced into one of the two pre-mix parts of the adhesive. The concentration of particles is pre-determined so that when the two parts are mixed, the particles are of a sufficient concentration to provide the desired conductivity. Frequently, though, the mixture ratio of the two-part fluid varies due to disturbances in the mixing system and other sources of error. When the mixture ratio varies from the optimum, the concentration of the conductive particles also changes. As a result, the conductivity of the joint is affected. Likewise, the load carrying capabilities of the joint can change also.
In many applications, an automated mixer is used to mix batches of the two-part adhesive. Typically, the flow rate of each of the two parts of the mixture is determined at the beginning of the production run and again at the end of the run. While these spot checks detect some variations from the optimum mixture ratio, the spot checks do not continuously monitor the system. If a variation occurs between the initial and final checks, non-optimum adhesive can be created and applied to the joint(s) being made with the adhesive.
Because the mixture ratio influences the properties of the joint, monitoring the mixture ratio of two-part fluids in real time and on a continuous basis would improve quality and reduce post-cure inspection processes.